The invention relates to shock and vibration resistant tactical grade accelerometers, particularly high frequency surface acoustic wave (SAW) accelerometers.
Modem resonating beam force sensing technology is an accepted and widely-used in high precision accelerometer design. Such accelerometers, however, are relatively costly to manufacture, and are susceptible to damage from high shock or vibration loads. Presently, there are numerous applications for a lower cost, high precision accelerometer that is rugged enough withstand high g-loading associated with extreme shock and vibration. For example, gun-fired military munitions which typically experience firing shocks on the order of 15,000 to 20,000 g""s could be designed for greater accuracy using guidance systems contained within the munitions. An accelerometer within munitions must meet high g-load and high precision demands, while remaining inexpensive enough to be expendable. In another example, extremely high accelerations are experienced in an aircraft crash. Recording deceleration of an aircraft during a crash and the moments preceding could provide useful information in determining the cause of the crash. In yet another example, the impact of a meteorite on a spacecraft may typically provide accelerations far in excess of those a typical guidance accelerometer can sense and survive.
Generally, existing accelerometer and other displacement or strain sensing device encompass variety of structures and principles. Each approach has been found to have its particular merits; but many defects are also present, such as lack of sensitivity and reliability on the one hand, and fragility and high cost on the other. Some accelerometer arrangements, for example, require auxiliary equipment, such as feed back mechanisms for providing reliable calibration or a temperature compensation mechanism. Many such existing arrangements are inherently analog in nature and do not lend themselves directly to use in digital equipment. Mechanism size and weight often add additional constraints.
Surface acoustic wave (SAW) technology is used in devices such as oscillators and electronic filters. Such devices use a piezoelectric substrate to sense vibrations and convert sensed vibrations to electrical signals. Existing transducers employing SAW technology use relative changes in the acoustical propagation characteristics of surface waves traveling on opposite surfaces of a thin elastic cantilever beam to directly measure the degree of flexing or surface strain of the elastic member.
SAW transducers commonly employ a flexible cantilever formed of a suitable piezoelectric material, such as Y-cut quartz. In general, piezoelectric materials are operationally reversible in that an applied mechanical strain will produce an electrical output and in that any electrical signal input will produce a related mechanical strain effect in the piezoelectric material. As disclosed by Cullen in U.S. Pat. No. 4,346,597 entitled, DUEL RANGE, CANTILEVERED MASS ACCELEROMETER, issued Aug. 31, 1982, the complete disclosure of which is incorporated herein by reference, the cantilever is clamped at one end, and a relatively substantial proof mass is affixed at the undamped end. Acceleration or other physically applied forces directed to the proof mass at the undamped end flex the cantilever. Thus, the cantilever, which forms the basic structural member, does not significantly contribute to the sensitivity of the apparatus. Rather, the cantilever merely transmits the stress developed by forces on the proof mass to a sensing element affixed to a flexing surface of the cantilever. Measurement of the frequency of a SAW resonator electrode pattern affixed to the flexing surface yields a measure of strain of the cantilever resulting from forces applied to the proof mass.
SAW devices are known to generate an output signal that varies in frequency as a function of strain, rendering it easily adapted for use with digital processing circuits. Therefore, accelerometers employing surface acoustic wave effects are desirable in applications where digital computation is necessary, and real time processing delay constraints preclude conversion of various analog signals to digital form. Elimination of analog to digital converters is particularly desirable in applications such as guided munitions having weight, space and cost constraints.
Piezoelectric materials are also generally pyroelectric in nature: any change in the temperature of the piezoelectric material produces a corresponding electrical output. SAW devices are thus thermally sensitive in that relatively small changes in the ambient temperature generate electrical outputs that are sufficiently large enough to cause spurious readings from the accelerometer. Therefore, as disclosed by Kellet in U.S. Pat. No. 5,063,782, entitled ACCELEROMETER AND ASSOCIATED CONTROL CIRCUITS, issued Nov. 12, 1991, the complete disclosure of which is incorporated herein by reference, many existing SAW accelerometers are suitable only for use at low frequencies and in conditions where the significance of ambient temperature changes upon the output of an accelerometer is limited.
Therefore, there is a real need in many high impact and vibration applications for the features of an accelerometer which are achievable essentially only by an expendable, digital-compatible, highly force sensitive, yet thermally insensitive, SAW type of strain sensor.
The present invention is a resonating beam accelerometer that overcomes the limitations of the existing devices by providing a thermally insensitive, expendable, digital-compatible, highly force sensitive surface acoustic wave (SAW)-type strain sensor micromachined as a monolithic device with an integrally-formed, cantilever-style detector.
According to one aspect of the invention, the SAW accelerometer of the invention, the fragile vibrating tines of the existing devices are eliminated. Instead, the SAW accelerometer of the invention detects vibrations on the surface of a relatively robust crystalline block. In addition, according to the present invention, substantially identical patterns of surface acoustic wave delay lines or resonator electrode patterns are deposited directly on opposing surfaces of a resonating flexure portion the integrally formed cantilevered beam, thereby resulting in a monolithic device requiring few manufacturing steps. High performance with low manufacturing cost is thus achieved. Also, the SAW accelerometer is designed for use with standard common mode rejection techniques that will eliminate the effects of temperature, pressure and vibration on accelerometer performance.
According to one aspect of the invention, the SAW accelerometer of the invention provides a high nominal operating frequency that results in superior performance values over current accelerometer devices.
According to other aspects of the invention, the invention provides a monolithic high frequency surface acoustic wave (SAW) accelerometer formed of a frame formed in a substrate of piezoelectric material; an integral beam suspended by one end from an interior surface of the frame; substantially identical patterns of surface acoustic wave delay lines formed on opposing surfaces of the beam; and a digital processing circuit coupled to each of the patterns of surface acoustic wave delay lines.
According to one aspect of the invention, the patterns of surface acoustic wave delay lines are formed at a location on the beam adjacent to the suspended end, and the beam is optionally further formed with an integral proof mass portion spaced away from the suspended end. According to another aspect of the invention, the proof mass portion is dimensionally larger than the portion having the patterns of surface acoustic wave delay lines formed thereon, thereby increasing the sensitivity to a force, i.e., acceleration, input.
According to yet other aspects of the invention, a method for forming an accelerometer is provided, the method including forming a frame of a piezoelectric material; forming a cantilevered beam-shaped sensing member of the piezoelectric material integrally with the frame; depositing substantially identical patterns of surface acoustic wave delay lines on opposing surfaces of the beam; and connecting a circuit to each of the patterns of surface acoustic wave delay lines, the circuit adapted to apply a high frequency drive signal to each of the patterns of surface acoustic wave delay lines.
According to one aspect of the invention, the method further includes connecting a frequency detection circuit to each of the patterns of surface acoustic wave delay lines. Preferably, the circuit adapted to apply a high frequency drive signal to each of the patterns of surface acoustic wave delay lines includes the frequency detection circuit as a subpart thereof.
According to still another aspect of the invention, the method further includes optionally mass loading the sensing member with an integrally formed proof mass.